SAR (Specific Absorption Rate)In general, exposure standards are based on a measurement called the
"specific absorption rate" (SAR).

The specific absorption rate is defined by the Institute of Electrical
and Electronics Engineers (IEEE) as:

'the time derivative of the incremental energy (dW) absorbed by (dissipated
in) an incremental mass (dm) contained in a volume element (dV) of
a given density'.

The specific absorption rate is defined by the ANSI standard reads:

'SAR is the time rate at which radio frequency electromagnetic energy
is imparted to an element or mass of a biological body.' SAR is expressed
as energy flow (power) per unit of mass in units of W/kg'.

When referring to human tissue, this means that the SAR is a measurement
of the heat absorbed by the tissue.

Measuring SARMeasuring specific absorption rates relies upon a simple process in
theory, but a nearly impossible procedure in practice because of the intrusive
nature of the ideal methodology.

In practice, the SAR is measured directly as a temperature increase
in a localized area of tissue. To do this it is necessary to insert
calorimetric probes into a live cell phone user's head in order to
map SARs directly.

As a result model heads and mathematical simulations of exposed heads
seem the only viable options for estimating SARs. However building a model
head inherently involves approximations in tissue simulation and model
complexity. Similar problems exist for computer models. Thus, there is
considerable variation across different SAR studies.

Maximum Permissible ExposureThere is considerable debate over cellular telephone Maximum Permissible
Exposure (MPE) standards - the maximum level of electromagnetic radiation
to which humans should be exposed. There are therefore a wide range
of MPE levels which are used in different countries. I am using an FCC
definition here.

Normally, Maximum Permissible Exposures are based on power density
measurements (milliwatts per square centimeter) to determine compliance.
Unlike SARs, power densities are directly measurable using field probes
and meters.

The FCC sets a maximum power density of 4mW/cm² for devices
that operate in the frequency band as cellular phones. But the standard
says that power density measurements can only be used for devices that
operate at a distance greater than 20 centimeters from the human body.
Devices operating within 20 centimeters of the human body, such as cellular
phones, must use the SAR of 1.6 W/kg.